1. Field
The present invention relates to wireless communication systems generally and, specifically, to methods and apparatus for reducing the impact of multipath interference on pilot estimation.
2. Background
In a wireless radiotelephone communication system, many users communicate over a wireless channel. The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. However, the spread spectrum modulation technique of CDMA has significant advantages over these modulation techniques for multiple access communication systems.
The CDMA technique has many advantages. An exemplary CDMA system is described in U.S. Pat. No. 4,901,307, entitled “Spread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeaters”, issued Feb. 13, 1990, assigned to the assignee of the present invention, and incorporated herein by reference.
An exemplary CDMA system is further described in U.S. Pat. No. 5,103,459, entitled “System And Method For Generating Signal Waveforms In A CDMA Cellular Telephone System”, issued Apr. 7, 1992, assigned to the assignee of the present invention, and incorporated herein by reference.
In a relatively noise-free data communication system, when data is transmitted over a communication channel by means of a linear modulation scheme, for example by using Quadrature Phase Shift Keying (“QPSK”), the number of detectable-levels that the channel can support is essentially limited by Inter Symbol Interference (“ISI”). ISI arises because of the “spreading” of a transmitted symbol pulse due to the dispersive nature of the channel, which results in an overlap of adjacent symbol pulses. Stated differently, ISI occurs when a portion of a signal representative of one transmitted pulse interferes with a different portion of the signal representative of a different transmitted pulse.
The adverse effects of ISI are more pronounced where the signal to noise ratio is high and the channel is relatively noise-free. In such channels, which are typically more important in data (as opposed to voice) communications, the presence of ISI greatly degrades performance of the communications system.
A common cause of ISI is the “multipath” phenomenon. Simply stated, multipath refers to interference caused by the reception of the same signal over multiple paths. Depending on the environment surrounding the mobile station (also called the “subscriber unit”) such as the existence of buildings or mountains, copies of the transmitted symbol pulses may arrive at the receiver at different times. As such, components of neighboring symbol pulses may interfere constructively or destructively.
It is generally known that equalization can be used to minimize the effects of ISI. Equalization involves altering a signal so that it may be more easily recognized at the receiver. A signal may be altered at the transmitter so that the influence of the channel on the signal will yield a signal capable of being properly recognized at the receiver. However, transmitter-based equalization is difficult since the transmitter must have a priori knowledge of the characteristics of the channel and any changes that may occur to the characteristics of the channel over time.
Equalization may also be performed at the receiver. Receiver-based equalization can use properties of the received signal to adjust equalization parameters. In wireless communications, since the mobile channel is random and time varying, equalizers must track the time varying characteristics of the mobile channel. Equalization attempts to apply a desirable amount of correction to the channel.
Receivers in mobile stations generally benefit from utilization of an equalizer. Conventionally, time domain equalizers have been used but they are in less effective if the mobile station is moving at a velocity greater than 10-20 kilometers-per-hour. Frequency domain equalizers are also known and provide faster channel adaptation capability (improved convergence time). While frequency domain equalizers are desirable for timely convergence and operability over increased mobile station velocities, frequency domain equalizers greatly benefit from the use of a specific form of a transmitted signal that is not present in a conventional CDMA forward link (“FL”) channel. Such specific formats could be attained in ‘unconventional’ CDMA forward link channels by inserting cyclic prefix or ‘unique word’ in the transmitted signal.
In a deployed communication system, a substantial alteration to the forward link channel would render obsolete the existing or “legacy” mobile stations that are not specifically configured to interact with the substantially altered forward link signal. Therefore, since base stations transmit to a variety of mobile stations, the transmitted or forward link FL signal must be compatible with existing or legacy mobile stations while also providing a signal configured to facilitate equalization in mobile stations that include frequency domain equalizers.
Therefore, there is a need to provide a transmitted signal that accommodates both operation of the legacy mobile stations while supporting the incorporation and operation of equalizers in equalized mobile stations.